The Histogram

A histogram displays all the pixels in an image as if laid out in a row from the darkest on the left to the lightest on the right. The peaks on the graph indicate there are lots of pixels of a certain brightness range, which is why an image dominated by dark tones will have a histogram stacked to the left, and an image of light tones will be heavily biased to the right. The histogram helps you judge your Levels adjustments.